1. Field of the Invention
The invention pertains to the field of switches for pulsed power applications. More particularly, the invention pertains to switches for pulsed power applications requiring high voltage blocking, high current density, compact size, low cost, high reliability, high efficiency, and fast current rise times.
2. Description of Related Art
Current commercial technology provides many semiconductor devices for switch applications. These include but are not limited to thyristors, such as the gate turn-off (GTO), metal-oxide-semiconductor gated (MOS-Gated) thyristor, and integrated gate-commutated thyristor (IGCT), and transistors, such as the insulated gate bipolar transistor (IGBT) and the metal-oxide-semiconductor field-effect transistor (MOSFET). Some of these devices have very fast turn-on times, very low resistance, or high voltage blocking capabilities. But none of them have the parameters required for some pulsed power applications, such as the ability to achieve 5 kV blocking, 5 kA peak with a turn-on time of less than 50 ns, and an on-state resistance less than 10 mΩ, at a reliability and price suitable for sale in the commercial market.
A device with the lowest on-state resistance and highest current density is a thyristor. The thyristor is basically a four layer device having alternating doping structure in the form P-N-P-N. The first P is connected to the anode metal and the last N is connected to the cathode metal. Typically the second P is connected to the gate metal for silicon (Si) devices. For asymmetric devices, those which block only in one direction, the first N is the only thick layer.
The turn-on time of the device, when electrically controlled, is limited by the thickness of the layers, the shape of the gate structure, and the gate current. The thickness of the layers determines the transit time of the charge carriers which directly impacts the turn-on time. The greater length of the gate-cathode region per active area, the faster the turn on time. The interdigitated structure can achieve meters of gate-cathode length per cm2 of active area, which is several orders of magnitude better than more common involute structures. These properties have been maximized in devices like the Solidtron, a GTO type thyristor, from Silicon Power Incorporated. The result is a device with a turn-on time greater than 200 ns. A slower device results in increased turn-on losses which means lower switching efficiency and higher thermal load.
Faster devices are being developed in two areas. One area is concerned with trying to develop faster devices with improved efficiency in power transmission or conversion. This area needs faster devices to reduce the turn-on losses, which become the more dominant loss factor as the on-state resistances have become very low. These devices can operate at very high frequency and average current, but have low peak currents.
The second area, to which this invention is applicable, is concerned with low-frequency, high-peak power applications. These applications include military defense, medical, and research fields. More specific examples could be radar, radiation therapy, or particle accelerators.
Based on the need for faster devices, much work is being directed toward creating switches for pulsed power applications. Some of that work involves using silicon carbide (SiC), gallium nitride (GaN), or other materials that can handle higher electric fields and therefore would have inherently faster operation. However these devices have very long development times and also high cost compared to standard Si devices. Many advances have been made with silicon devices improving turn-on time such as using highly interdigitated gate structures, laser triggering, and modified structures. One further method for improving turn-on time is optical pumping such as described by U.S. Pat. No. 4,207,583.
Laser pumping uses photons to generate charge carriers in a semiconductor device. Because of the regenerative properties of thyristors, once sufficient charge carriers are generated, the regenerative action will maintain sufficient charge carriers to conduct the current at the on-state resistance. So, optical pumping can allow a semiconductor device to turn-on as fast as the charge carriers can be photo-generated, rather than waiting for those charge carriers to be injected from the base region as is required for electrically or optically triggered semiconductor devices.
A limitation with laser pumping has been the cost and size of the required laser source. Previously solid state or gas lasers operating in the infrared (IR) range were used. These types of lasers are large and costly and were only usable in installation having sufficient space and resources. To be able to commercialize the devices, high-power, low-cost, pulsed, compact laser diode arrays needed to be developed. Normally laser diodes operate in or near continuous-wave (CW) operation at low peak power for applications like fiber-optic communication. Also, previously laser diodes were not produced with wavelengths appropriate for laser pumping Si thyristors.
Other attempts at laser pumped devices used thick silicon which is not a commercial material. Thick silicon also makes it harder to optimize the distribution of charge carriers throughout the device, as there is an imbalance between absorption and photon energy. If the laser wavelength is too long then the energy is below the band-gap and the photon cannot efficiently generate charge carriers. If the wavelength is too short then all of the light is absorbed in a thin layer and the charge carriers are not photo-generated throughout the device.